The Dornier Do 17 became obsolete very soon after the
start of WWII and was replaced by the Do 217. It was used how ever by
the DFS (Deutusche Forschungsinstitut für Segelflug) to test Lorin
type ramjets of various sizes being designed by Dr Eugen Sanger from 1941
onwards, an increase in speeds of over a 150km/h were recorded. With the
strain being put on the Do 17's airframe it was decided to use a Do 217E
to test the bigger engines in the development of a ramjet engine needed
to power a supersonic aircraft. By 1945 Dr Sanger had designed the SK
P.1040 which was just being built by the end of the war.

Two other companies and designers were also working on
ramjet type engines: Argus
and Lippisch

A nice side on view of the
Do17z showing the fittment of the Lorin Ramjet

Do 217E fitted with a Sanger ramjet

The same Do 217E from the rear

Other Tests using the Do 217

Do 217K with the DFS 228

Also the high altitude rocket
powered research aircraft the DFS 228 used the Do 217 as a mother
plane, mounting the DFS 228 in the same way as the Me 328.

As well as testing engines the Do 217 was used to
test a number of prototype aircraft such as the Messerschmitt
P.1073 (Me 328) light fighter, with the little Messerschmitt being
mounted "piggyback" fashion the mother plane.